1. Field of the Invention
The present invention relates to a method of growing a germanium (Ge) epitaxial thin film required for fabricating a germanium (Ge) photodiode, and more particularly, to a method of growing a germanium (Ge) epitaxial thin film having superior characteristics of relaxed stress, low penetrating dislocation density, and reduced surface roughness on a silicon substrate using reduced pressure chemical vapor deposition (RPCVD), and a photodiode using the same.
2. Discussion of Related Art
A new device using a silicon (Si) and silicon-germanium (SiGe) or germanium (Ge) heterojunction structure has been developed in the last several decades.
In particular, a high-speed heterojunction bipolar transistor (HBT) using a difference in band-gap energy between silicon (Si) and germanium (Ge), a dual-channel of a modulation doped field effect transistor (MODFET) using high mobility characteristics exhibited in a stress-applied Si/Ge thin film or a metal oxide semiconductor field effect transistor (MOSFET) are mainly being fabricated.
Further, research into fabricating a photodiode for optical communications in a region ranging from 1.3 to 1.55 μm that is an optical absorption wavelength band of germanium (Ge) band-gap energy (0.67 eV) has recently been under way. For this purpose, a method of obtaining a Si—Ge buffer layer exhibiting excellent characteristics or a pure germanium (Ge) thin film on a silicon substrate is being researched, and results thereof are being reported.
A lattice strain f that is a major parameter in a Si/Ge heterojunction structure may be represented by the following equation.f=(alayer−asub)/asub 
Here, asub denotes a lattice constant of a substrate, alayer denotes a lattice constant of a deposited layer, and a lattice strain f between silicon (Si) and germanium (Ge) is 4.2%. Further, different equilibrium critical thicknesses tc are given according to a deposition temperature of germanium (Ge). As the deposition temperature of the germanium (Ge) is increased, tc decreases. When the thickness is less than tc, a lattice structure exists in a stable state by elastic deformation to a square, whereas when the thickness is greater than or equal to tc, energy required to produce a misfit dislocation becomes smaller than elastic deformation energy of the deposited germanium (Ge), and a stress relaxation phenomenon occurs due to the misfit dislocation. tc depends on a nucleus production position or propagation mechanism of the dislocation as well as a deposition temperature.
Since a germanium (Ge) epitaxial layer is a part in which an active device is to be formed in a germanium (Ge) photodiode, the germanium (Ge) epitaxial layer must satisfy several requirements. First, a stress resulting from the misfit dislocation should be sufficiently relaxed. Second, a surface of the thin layer should be smooth. A rough surface with a wave pattern may be an obstacle to a process of fabricating a device. Third, a penetrating dislocation which may occur while the stress is relaxed should not be propagated to a surface of the germanium (Ge) thin film. The penetrating dislocation which is propagated to the surface may serve as a defect in a thin film to be formed thereon, and device characteristics may be deteriorated or a leakage current may be generated.
However, the existing adjusted diffraction factors may function to enable carriers to cause transition effects between energy valleys, and thus negative resistance effects may occur. An optical device exhibiting such negative resistance effects may have fast frequency response characteristics. For example, as disclosed by G. Kim et al. (Appl. Phys. Lett. 83, 1249, 2003), it has been reported that a high-performance photodiode based on a group III-V semiconductor exhibits enhanced frequency response characteristics based on negative photoconductance.
Finally, in order to accomplish commercialization, a process of a germanium (Ge) epitaxial thin film should be simplified to reduce a process time.
A conventional technique includes a two-stage growth method including high and low temperatures, and it is reported that after the two-stage growth method, a method in which repeating annealing at a temperature of 780° C. for 10 minutes and at a temperature of 900° C. for 10 minutes ten times is a germanium (Ge) epitaxial growth method that is most likely to exhibit excellent characteristics. This process is performed using costly ultra-high vacuum chemical vapor deposition (UHVCVD) equipment that is unsuitable for commercialization, and takes four hours to complete only a post-annealing process. Also, the annealing performed at a high temperature may cause mutual diffusion between silicon (Si) and germanium (Ge), and thus a Si—Ge layer is formed on an interface thereof. This changes a band-gap and influences device characteristics.